Microdialysis: Touching the fingertips of the cardiac sympathetic nervous system

For years sympathetic activity in man and experimental animals has been assessed by measuring circulating catecholamines. As their plasma concentration is not only determined by sympathetic activity but also by spillover and clearance, it is only useful as a screening tool for gross disturbances in general sympathetic tone. The isotope dilution method uses tritiated norepinephrine to differentiate between norepinephrine release into and its removal from the circulation. This technique is not only more precise but can also be applied for the assessment of regional sympathetic tone. Alternatively, local sympathetic activity can be monitored by microneurography, i.e. measuring the electrical activity of postganglionic sympathetic efferents. However, due to its invasive nature, microneurography in humans is restricted to monitoring sympathetic control of skin and muscle vasculature. Microdialysis is a new technique that can monitor local sympathetic act!Vlty almost continuously by measuring interstitial norepinephrine concentrations. The technique is based on the diffusion of norepinephrine from the intercellular space through a semi-permeable membrane mounted in a small catheter into a suitable perfusion fluid like Ringer's or Ringer's lactate, which can be collected continuously for later analysis. In conclusion, none of the mentioned techniques for monitoring sympathetic activity is superior to another as each has its own strengths and limitations. The choice for one or more of these methods strongly depends on the question that has to be answered; a combination of the various techniques may provide the investigator with a more powerful tool to monitor the sympathetic nervous system.

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